1. Field of the Invention
The present invention relates to a high-frequency heating apparatus for irradiating a microwave so as to perform dielectric heating, and it particularly relates to a microwave oven.
2. Description of the Prior Art
In a high-frequency heating apparatus, there is a method for heating to heat an object uniformly where the position of the object to be heated relative to a standing wave in a heating chamber is constantly change. Thus, a portion of the object to be heated thereby changes, so as to realize a uniform heating. For example, a stirrer fan is conventionally used for rotating a metal fan mounted on a ceiling, a side wall or a base so as to stir the microwave, a turntable method for heating the object while the object is being rotated, and a rotation antenna method for rotating an antenna which is a radiator of an electric wave.
FIG. 1 shows a high-frequency heating apparatus employing the turntable method. In the high-frequency heating apparatus shown in FIG. 1, there is provided at a side wall 3 a magnetron for generating the microwave, and the microwave thus generated from the magnetron is radiated from an excitation opening 9 provided at an upper portion of the side wall 3 into a heating chamber 1 through a wave guide 7. Then, the irradiated microwave reaches an object 13 placed on a rotating turntable 11, so as to heat the object.
In such a conventional high-frequency heating apparatus, the object 13 to be heated is rotated by the turntable 11. The microwave radiated from the above evenly irradiates the object 13, to realize a uniform heating. For solid and semi-solid objects, almost uniform heating can be obtained. However, in the case of a container filled with a liquid , there occurs a temperature difference between an upper portion and a lower portion therefor due to heat convection, so that there is heat unevenness in the liquid. For example, referring to FIG. 2, the temperature difference is significant in heating a sake in a sake bottle or a cup of milk or the like. This undesirable temperature difference occurs not only in the turntable method but also in the stirrer fan method as well as the rotating antenna method where in these methods the excitation opening for the microwave is provided in the upper portion of the heating chamber 1.
FIG. 3 shows a temperature rise at each point A through D when the sake bottle is filled with water and dielectric-heated. In the same figure, a rate of temperature rise in the upper portion of liquid is faster than that in the lower portion of liquid, thus causing a temperature difference therebetween to become greater as time lapses. For example, suppose that 40.degree. C. is a proper temperature for heating sake. Then, by the time the lowest portion D of the sake bottle becomes a temperature of 40.degree. C., there is a temperature difference of over 15.degree. C. between D and the upper position A. At this stage, sake in a certain portion of the bottle presents a desirable temperature while other portion thereof does not.
Accordingly, there is a problem in the conventional high-frequency heating apparatus where a significant temperature difference is caused by a liquid load in heating the liquid.
In the high-frequency heating apparatus it has, in general, a single excitation opening. A wave guide switching device is not required for such the apparatus with a single excitation opening. Furthermore, even for a microwave oven having a plurality of excitation openings there is no wave guide switching device.
However, considering the various loads in a microwave oven, it would be more efficient if the excitation openings are switched according to each load, so as to achieve an optimum heating of the load.
Accordingly, the conventional microwave oven is not equipped with the wave guide switching device. Moreover, even if a usual wave guide switching device is implemented for the microwave oven, a cost increase, heavier weight thereof and a bulky size, result. These are not suitable for the microwave oven.
When various loads are heated by the microwave oven, it is desirable to have a plurality of magnetrons in order to achieve the optimum heating of the load. For example, FIG. 5 shows a microwave oven with which two magnetrons are equipped.
In a microwave oven having a plurality of magnetrons, there are conventionally provided separate power supplies for each magnetron in order to drive the magnetrons. Thus, each power supply is switched on and off separately in order to switch the excitation opening for each magnetron.
FIG. 4 shows a schematic diagram of a power supply portion which drives two magnetrons. The primary sides of the two magnetrons 51, 53 are connected to transformers 57, 59, which are, in turn, connected to an a.c. power supply 55. More specifically, an anode of the magnetron 51 is connected to a high-voltage secondary winding 57a through a voltage doubler rectifying circuit, and a filament of the magnetron 51 is directly connected to a filament-use secondary winding 57b of the transformer 57, thus a high voltage and a filament voltage being supplied thereto respectively. In the similar manner, an anode of the magnetron 53 is connected to a high-voltage secondary winding 59a through a voltage doubler rectifying circuit, and a filament of the magnetron 53 is directly connected to a filament-use secondary winding 59b of the transformer, thus a high voltage and a filament voltage being supplied respectively. Primary sides of the transformers 57, 59 for the magnetrons 51, 53 are respectively connected to the a.c. power supply 55 through switches 61, 63 so that the magnetrons 51, 53 are switched on and off by the switches 61, 63, respectively.
However, in the above conventional microwave ovens, there are provided respective power circuits including transformers and rectifying circuits and-so on for each of the plural magnetrons, thus causing a problem of being not economical and of occupying a large space for mounting thereof.